Our invention relates to a power line protection circuit, and particularly to a power line protection circuit for delaying trip signals under selected noise conditions.
Frequency-shift-keyed signals are used extensively in 60 Hertz power line systems where line conditions are indicated by frequency-shift-keyed signals. For example, a first power station may supply power over a transmission line to a second station. The second station monitors its loads and other outgoing lines, and as long as these loads and lines are in good condition, the second station sends a first frequency guard signal to the first station. As long as the first station receives this guard signal, it supplies power over the line to the second station. But if the second station detects an improper condition, such as a fault on one of its outgoing lines, the second station removes the guard signal and sends a second frequency trip signal to the first station. Upon loss of the guard signal and upon receipt of the trip signal, the first station trips (or opens) the transmission line to the second station, thereby protecting the equipment at the first station. Because of modern society's need for electrical power, the decision to maintain or open an electrical power line is very important.
Accordingly, a primary object of our invention is to provide a new and improved protection circuit for use in the control of electrical power lines.
Usually, the guard and trip signals are sent and received by carrier frequencies over telephone lines or radio circuits. While telephone lines or circuits are reliable, they are subject to noise. But whatever facility or circuit may be used, its connection or utilization at a 60 Hertz power station makes it inherently noisy from an electrical viewpoint. And when there is a 60 Hertz power or equipment malfunction, the communication facility or circuit may become even noisier, just at the time reliable guard and trip signals are most needed.
Accordingly, another object of our invention is to provide a new power line protection circuit that provides improved operation even under very noisy electrical conditions.
Another object of our invention is to provide a new and improved power line protection circuit that delays a trip signal under certain noise amplitude conditions.
In a protection circuit for 60 Hertz power line systems, the absolute time required for a trip signal to be transmitted, received, and produced is an important design consideration. Of equal importance are the security (against a false trip) and the dependability (for a real trip) of the protection circuit. One prior art protection circuit increased the security (against false trips) with an amplitude squelch circuit and an added fixed time delay in the trip tone receiver. However, the squelch circuit decreased the dependability (for a real trip), and the added fixed time delay decreased the response speed.
Accordingly, another object of our invention is to provide a new and improved power line protection circuit that delays production of a trip signal only when selected noise conditions exist.
Another object of our invention is to provide a new and improved power line protection circuit that utilizes wideband and narrowband noise sensing circuits, and that delays production of a trip signal only when selected conditions exist in either the wideband or the narrowband noise sensing circuits.